**Core Concept**
The clinical presentation of an annular plaque with peripheral scale in a child suggests a fungal infection, likely tinea corporis. The use of topical nystatin, an antifungal agent, indicates a possible misdiagnosis or inadequate treatment.

**Why the Correct Answer is Right**
Topical nystatin is not effective against dermatophytes, the causative organisms of tinea corporis. The peripheral scale is a characteristic feature of dermatophyte infections, which are resistant to nystatin. The correct approach is to use a topical antifungal agent containing an imidazole (e.g., clotrimazole or miconazole) or an allylamine (e.g., terbinafine), which are effective against dermatophytes.

**Why Each Wrong Option is Incorrect**
**Option A:** Oral antifungal agents like fluconazole may be used in severe cases or for widespread infections, but are not the next best step in this scenario.
**Option B:** Bacterial cultures are not indicated in this case, as the clinical presentation is consistent with a fungal infection.
**Option C:** Biopsy and histopathological examination may be useful in cases of unclear diagnosis, but are not the next best step in this scenario.

**Clinical Pearl / High-Yield Fact**
Dermatophytes are a group of fungi that cause skin, hair, and nail infections, and are typically resistant to polyenes like nystatin.

**Correct Answer:** C.